What these guys are saying about the fuel pump, air leaks, squeezing the primer bulb when it misbehaves, etc. are all things to check first. But I’ll throw this out there only because of the symptom you describe as repeatedly slowing down and speeding up. There are a couple variations of that symptom but one of the causes can be electrical.
Some guy sent me a video of what his ’63 Evinrude 10 HP was doing and when I posted it here a while back a couple guys immediately said, “that looks like an electrical problem.”
So I popped the flywheel and saw a black mark on the armature plate where there was some arcing going on. So to avoid any hassle I just replaced the coil and plug wire and the motor ran great after that.
Not saying this is your problem, just passing it along as an outside chance that your slowing down and speeding up is an electrical issue somewhere.
